Past work of this group has demonstrated that an osteoclastic protein-tyrosine phosphatase (PTP-oc) is a positive activator of the osteoclast activity. Recent preliminary studies have disclosed four exciting discoveries relevant to the mechanism of how PTP-oc expression in osteoclasts is regulated: First, PTP-oc expression is controlled primarily by post-transcriptional mechanisms. Second, the post-transcriptional regulation of PTP-oc expression in osteoclasts is regulated in part by miR17. Third, the miR-17 expression in mature osteoclasts is controlled by certain resorption regulators that also regulate cellular PTP-oc levels in osteoclasts; and fourth, conditional deletion of the miR17~92 gene cluster in osteoclastic cells resulted in not only an increase in PTP- oc mRNA level but also increases in the cell size, number of nuclei, fusion, and bone resorption activity of osteoclasts. These ground-breaking discoveries led to the formulation of three novel hypotheses: 1) Certain resorption cytokines, such as PTH, IL-1, 1,25(OH)2D3, PGE2, activates osteoclasts by reducing miR17 transcription through suppression of its promoter activity; 2) conditional deletion of miR17 in osteoclasts increases expression of PTP-oc, which in turn stimulates osteoclast activity via up-regulation of the PTP-oc signaling; and 3) conditional overexpression of miR17 in osteoclasts suppresses PTP-oc expression and inhibits physiologically and pathologically-induced bone resorption. The first Aim tests the first hypothesis by a) determining if treatment with selected resorption modulators regulate the promoter activity of miR17 in osteoclasts in vitro; b) identifying and mutating essential response elements to demonstrate direct effects of these resorption modulators on miR17~92 gene transcription; and c) performing ChIP-seq analysis to identify essential transcription factors that are involved in the transcriptional regulation of the expression of miR1 in osteoclasts. Aim 2 tests the second hypothesis by characterizing the in vivo and in vitro bone and osteoclast phenotypes of osteoclast miR17~92 conditional knockout mice, determining effects of miR17~92 deficiency on PTP-oc mRNA level, the PTP-oc signaling, and resorption activity of osteoclasts in vitro, and determining whether re-introduction of miR17 into miR17~92 deficient osteoclasts would restore the osteoclast phenotype in vitro; b) evaluating effects of miR17~92 deficiency on osteoclast differentiation by performing RNA-seq gene expression profiling and pathways analyses and by determining the role of Rac1/Rac2 in the miR17-deficiency- induced osteoclast fusion; and c) showing that the observed defective bone coupling in conditional knockout mice is not due to deletion of miR17 in osteoblasts, reduced osteoblast activity, but instead to an impaired bone cell production of coupling factors. Aim 3 tests the third hypothesis by generating transgenic mice with conditional overexpression of miR17~92 in osteoclasts by crossing LysM-Cre mice with mice harboring the miR17~92 transgene downstream to a loxP-flanked Neo-STOP cassette at the ROSA locus, determining the effects of conditional overexpression of miR17~92 in osteoclasts on bone and osteoclast phenotypes in vivo and in vitro, and determining if overexpression of miR17~92 in osteoclasts would blunt the ovariectomy-and calcium depletion-induced bone resorption and bone loss. This research addresses an innovative concept that a miRNA (i.e., miR17) is a key regulator of the functional activity of mature osteoclasts through modulation of expression of PTP-oc in osteoclasts. If successful as expected, this work will yield important insights into how PTP-oc expression is regulated, which is critically important not only with respect to the overall understanding of the role of PTP-oc and its signaling mechanisms in the pathophysiology in various types of bone-wasting diseases, including osteoporosis. It could also provide novel targets (i.e., miR17) for future screening of small molecular therapeutic compounds as a rational design for an effective means to modulate PTP-oc expression and the functional activity of osteoclasts, which can then be used to develop novel and effective anti-resorption therapies for certain subtypes of osteoporosis and bone-wasting diseases.